Until recently, the magnetic fields used in hard disk drives to access data were in a direction essentially parallel to the accessed disk surface, which is referred to as the longitudinal direction. This approach is being supplanted by an approach known as perpendicular recording. It offers greater recording densities using magnetic fields that have lines of force perpendicular to the disk surface. Such hard disk drives are called Perpendicular Magnetic Recording (PMR) hard disk drives.
Perpendicular magnetic recording is inherently susceptible to external magnetic fields. This increased sensitivity to stray fields originates from the read head interacting with a Soft Under Layer (SUL) between a PMR layer and the disk substrate. These fields particularly affect desktop/mobile products, where a magnetic bracelet on someone's wrist can come close to the hard disk drive, disrupting its access operations. Further, elevators, automobiles, trains, ventilators, and air conditioning units can also magnetically disrupt operations of a PMR hard disk drive.
These external magnetic fields may distort writing and readback signals and cause error events. Not only recorded information may be lost, but also signals for identifying tracks and servo positioning to find the tracks may be lost. High current writing in the presence of strong longitudinal magnetic fields may cause large degradation of the Burst Error Rate (BER). Consequently, a PMR hard disk drive needs to be robust against stray fields in any direction with field strengths of up to 50 Oerstedt (Oe) under otherwise nominal writing conditions without needing to erase the data and without significant degradation of the BER performance.
Prior attempts to address this problem have included shielding the slider and use of the read head to estimate the longitudinal component of the external magnetic field. Both approaches have problems. Adding shielding requirements to the slider complicates a sensitive part of the PMR hard disk drive. Trying to guess the longitudinal component of the external magnetic field using the read head is questionable, since the read head is focused on sensing the perpendicular magnetic field off the disk surface. An economical and reliable approach is needed to monitor external magnetic fields within a PMR hard disk drive that can be used to protect it from strong longitudinal magnetic fields without requiring specialized magnetic shielding of the slider.